Continued growth in the electronics and computer industries, and indeed growth in the economy in general, is increasingly attributed to the demand for access to the Internet and myriad of services and features that it provides. The proliferation in the use of computing equipment, both of the conventional desk top variety as well as of the portable variety, including laptop computers, hand-held Personal Digital Assistants (PDAs), Internet enabled cellular telephones and other access devices have resulted in a corresponding increase in demand for network infrastructure.
The access points into the Internet are, however, mostly provided via communication systems that were originally intended for carrying non-data traffic. For example, the Public Switched Telephone Network (PSTN) is still heavily used as a dial-up access point for many home and personal users. Although there are emerging various standards that provide higher speed access points, these emerging technologies, as well as older high speed technologies such as TI and/or fractional TI services still make use of the telephone network. The telephone network was, unfortunately, optimized to carry voice traffic as opposed to data traffic. In particular, these networks were intended to support continuous analog communications, as compared to the digital communication protocols needed for Internet packet-oriented communications.
For example, voice grade services typically require access to a communication channel bandwidth of approximately 3 kilohertz (kHz). While techniques do exist for communicating data over such radio channels at a rate of 9.6 kilobits per second (kbps), such low bandwidth channels do not lend themselves directly to efficient transmission of data at the typical rates of 56.6 kbps or higher that are now commonly expected.
In addition, the very nature of Internet traffic itself is different from the nature of voice traffic. Voice communication requires a continuous duplex connection, that is, a user at one end of a connection expects to be able to transmit and receive to a user at the other end of a connection continuously, while at the same the user at the other end is also transmitting and receiving.
Usage patterns of the Internet are also quite different from voice communications. For example, consider that access to Web pages over the Internet in general is burst-oriented. Typically, the user of a remote client computer first specifies the address of a Web page to a browser program. The browser program at the client computer then sends the request as a Transmission Control Protocol (TCP)/Internet Protocol (IP) message packet, which is typically about 1000 bytes in length, to a network Web server. The Web server then responds by sending the content of the requested Web page, which may include anywhere from approximately 10 kilobytes to several megabytes of text, image, audio or video data. Because of delays inherent in the network, and because the Internet is such a vast interconnected mesh of networks, users experience delays of several seconds or more for the requested web page to be routed to them. The user may thereafter spend several seconds or even several minutes reading the contents of the page before specifying a next page to be downloaded.
The result is that a typical Internet connection remains idle for a relatively long period of time. However, once a request is made, the user expects the information to be transmitted to the client at a relatively rapid rate. An additional difficulty is provided in wireless access systems in that there are typically many more potential users or subscribers than the available number of physical radio channels. Therefore, making wireless channels available only on an instantaneous xe2x80x9cas neededxe2x80x9d basis makes sense, and indeed is a requirement if wireless data transfer services are to efficiently operate. Thus, dynamic traffic channel allocation schemes are one way to increase the efficiency wireless data communication systems in an effort to more efficiently utilize available channel resources.
Some type of demand-based multiple access technique is therefore typically required to make maximum use of the available wireless channels. Multiple access is often provided in the physical layer, such as by Frequency Division Multiple Access (FDMA) or by schemes that manipulate the radio frequency signal such as Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA). In any event, the nature of the radio spectrum is such that it is a medium that is expected to be shared. This is quite dissimilar to the traditional environment for data transmission, in which a wired medium such as a telephone line or network cable is relatively inexpensive to obtain and to keep open all the time.
A particular problem occurs in existing communication systems that use on-demand multiple access techniques to permit multiple users to share a physical channel. Due to the nature of Internet communications, these techniques increasingly make use of Time Division Multiplex (TDM) to assign time slots to specific users or connections on a demand basis. In such a system, time slot assignments are communicated to a receiver either explicitly or implicitly.
In an implicit assignment system, time slots are preassigned in a fixed pattern. Therefore, receivers know when to listen for data intended for them. However, implicit assignment systems are typically not flexible enough to efficiently handle Internet traffic.
In an explicit assignment system, time slots are assigned to specific users on a demand basis by a central system controller. Information as to which time slots are assigned to which connection is then explicitly communicated from the central controller to each remote unit. The overhead associated with transmitting information as to time slot assignment is therefore information bandwidth that otherwise cannot be allocated to transmitting payload data.
Unfortunately, this situation is exacerbated in a wireless communication environment in which additional radio channels must be allocated for communicating such time slot assignment information. This is a particularly acute problem on a forward link direction of such systems, that is in the direction from the network towards the user. Most Internet traffic is typically communicated in the forward direction.
The present invention seeks to overcome these difficulties. Specifically, the invention is used in a Time Division Multiplex (TDM) communication system where a physical radio communication channel, which may be defined by CDMA codes or in other ways, is shared among multiple users or connections through the use of time slots. Time slots are allocated on a demand basis. For example, a given radio channel in a forward direction is allocated only for a pre-determined time slot duration and only as needed by specific connections.
The invention overcomes certain disadvantages of prior art systems. In order to minimize overhead in the allocation of time slots to specific users, no specific time slot assignment information needs to be communicated to the receiver. However, time slot assignment may still be made on a demand basis.
This is accomplished through the use of a particular coding scheme at the transmitter, and a particular protocol at the receiver. The transmit coding scheme takes a data packet and divide it into sub-packets or frames. The frames are separately assigned to time slots at the transmitter, driven by connection demand. Each given frame is first encoded by a Forward Error Correction (FEC) code which may typically add additional bits to the frame. A user specific cover sequence, which may, for example, be a pseudonoise (PN) sequence, is added to the frame data. The FEC encoded frame is then assigned a time slot and transmitted over the shared radio channel.
At the receiver end of the connection, all receivers always attempt to receive to all frames in all time slots. As part of this receiving process, each receiver applies its specific assigned cover sequence in order to attempt to receive each frame. The candidate frame is then submitted to an inverse FEC decoding process to attempt to properly decode each frame.
A process within a first layer of the receive protocol, which may be implementation access layer, handle the candidate frame as follows. If a frame is properly decoded, as indicated by the FEC decoding process being successfully completed, the frame is passed up to the next higher layer of the receiver protocol. However, if a frame is erroneously decoded, it is simply discarded. Most importantly, the discarded frame event does not cause any error indication to be returned to a higher layer of the protocol.
The result is that only the receivers having the correct cover sequence assigned to them will properly decode frames intended for them. Any frames decoded that are not intended for that particular receiver will therefore normally be discarded.
A higher layer of the receive protocol then takes care of the problem of erroneously discarded frames intended for the receiver and/or erroneously accepted frames that were intended for other receivers. Specifically, the higher layer protocol may determine, from information contained in a frame such as a sequence number, when such frame has been erroneously discarded or erroneously accepted. Only at this higher layer, which may be a link layer of the protocol, will a receiver issue an error indication back to the transmitter, requesting re-transmission of the packet.
There are several advantages to this arrangement.
First, only cover code information, and not time slot information, needs to be made available at each receiver. Therefore, the overhead associated with dynamic assignment and deassignment of time slots to specific receivers, such as the need to transmitting information as to time slot assignment and deassignment is eliminated.
Second, the system works especially well where the system has wireless communication or other multiple access techniques, such as Code Division Multiple Access (CDMA), to define the physical channels. Eliminating the need to transmit time slot information from the transmitter to the receiver provides for much greater flexibility on demand assignment of individual channel resources. Reducing signaling overhead demand in such systems also increases the amount of information bandwith available for carrying payload data, while decreasing the amount of channel interference, thereby increasing capacity of the system as a whole.